1. Field of the Invention
The present invention relates to a semiconductor device and a fabrication process therefor and particularly, to a semiconductor device having an isolation region and a fabrication process therefor.
2. Description of the Background Art
As an example of a conventional semiconductor device, photo diodes applied in IC for an optical pickup of a compact disc (CD) will be described. As shown in FIGS. 25 and 27, a plurality of photo diodes PD11 to PD14 are first formed on a p-type silicon substrate 101. The photo diodes PD11 to PD14 are electrically isolated from one another by lower and upper isolation regions 103 and 105, strip portions of each of which intersect with each other in a plane.
In a region where the photo diodes PD11 to PD14 are formed, an n-type epitaxial layer 104 is formed on the p-type silicon substrate 101 with an N+-type buried region 102 interposed therebetween. On the n-type epitaxial layer 104, a p-type layer 106 is formed.
Electrons and holes are generated in a depletion layer expanded from the junction interface between the p-type layer 106 and the n-type epitaxial 104. Electrons are migrated into the n-type epitaxial layer 104 under an electric field in the depletion layer, while holes are migrated into the p-type layer 106, to thus produce a potential difference between the n-type epitaxial layer 104 and the p-type layer 106.
Further, similar to the above described case, a potential difference is produced between the N+-type buried region 102 and the p-type silicon substrate 101 by electrons and holes generated in a depletion layer expanding from the junction interface between the N+-type buried region 102 and the p-type silicon substrate 101. With such a potential difference, a photovoltage is produced. In such a way, a current generated in each of the photo diodes PD11 to PD14 is amplified by a predetermined circuit (not shown).
Then, of a fabrication process for the above described semiconductor device, especially a method, by which the lower and upper isolation regions 103 and 105 are formed, will be explained. First, in a predetermined region of the p-type silicon substrate 101, a p-type impurity used for forming the lower isolation region 103 is implanted by an ion-implantation method, followed by a predetermined heat treatment.
Then, the n-type epitaxial layer 104 is formed on the p-type silicon substrate 101. With such steps applied, as shown in FIG. 28, a region 103a to serve as a lower isolation region, is formed. Thereafter, a p-type impurity is ion-implanted on a surface of the n-type epitaxial layer 104 above the region 103a and a predetermined heat treatment is applied to produce a region 105a to serve as an upper isolation region 105, as shown in FIG. 29. Thereby, other transistors and so on, which are not shown in the figure, are formed on the p-type silicon substrate 101 to thus complete a semiconductor device having photo diodes.
However, in the above described fabrication process for a semiconductor device, there has been a problem described below. After the region 103a to serve as the lower isolation region 103 is formed, an impurity in the region 103a diffuses in all directions during a heat treatment to form the region 105a to serve as the upper isolation region 105.
Further, after the region 105a is formed, heat treatments during formation of impurity regions of other transistors not shown diffuse impurities of the region 105a in all directions. As a result of such diffusion, as shown in FIG. 25, in a semiconductor device completed after predetermined heat treatments, the isolation region in a section around an intersection of strip portions of the lower isolation region 103 or the upper isolation region 105 expands in size compared with its original one because of diffusion from adjacent parts to the section around the intersection.
On the other hand, a spot diameter of a laser light beam with which the photo diodes PD11 to PD14 are illuminated is limited and has a intensity distribution as shown in FIG. 26 for example. Electron-hole pairs generated by a laser light beam are in situ recombined since almost no depletion layer is formed in the isolation regions 103 and 105.
For this reason, a laser light beam with which the isolation regions 103 and 105 are illuminated contributes to almost no generation of a photovoltage. As described above, in the section around an intersection of strip portions of the lower isolation region 103 or the upper isolation region 105, the isolation region expands in size compared with its original one by heat treatments in a fabrication process.
At this time, when the isolation regions 103 and 105 are illuminated with a laser light beam such that illumination by a light beam spot A as shown in FIG. 25 covers the sections around an intersection of strip portions of the isolation regions 103 and 105, a light component contributing to generation of a photovoltage decreases and thereby, a problem arises since high accuracy information become hard to obtain by the photo diodes as an optical pickup device.
The present invention has been made in order to solve the above problems. It is an object of the present invention to provide a semiconductor device with a high photovoltage generation efficiency and it is another object of the present invention to provide a fabrication process for such a semiconductor device.
A first semiconductor device of an aspect of the present invention, including an isolation region having strip portions that intersect with each other in a plane, is provided with a first conductivity type semiconductor substrate, a second conductivity type layer and a first conductivity type layer. The second conductivity type layer is formed on the semiconductor substrate. The first conductivity type layer is formed on the second conductivity type layer. The isolation region is formed from a surface of the first conductivity type layer to a surface of the semiconductor substrate and partitions the first conductivity type and second conductivity type layers into a plurality of regions. Arms of strip portions of the isolation region are narrowed in width toward a center of an intersection of the strip portions in a section around the intersection.
According to the semiconductor device, since arms of strip portions of an isolation region are narrowed in width toward a center of an intersection of the strip portions in a section around the intersection, a depletion layer at the interface between the first and second conductivity type layers can expand more toward the intersection of the strip portions compared with a conventional semiconductor device. Further, similar to the above described case, a depletion layer at the interface between the semiconductor substrate and the second layer can expand more. With the expansion of the depletion layers, even when illumination with a laser light beam covers a section around an intersection of the strip portions, no electron-hole pairs generated in a depletion layer are in situ recombined under an electric field in the depletion layer expanding close to the section around the intersection, which can contribute to generation of photovoltage.
A second semiconductor device of an aspect of the present invention, including an isolation region having strip portions that intersect with each other in a plane, is provided with a first conductivity type semiconductor substrate, a second conductivity type layer and a first conductivity type layer. The second conductivity type layer is formed on the semiconductor substrate. The first conductivity type layer is formed on the second conductivity type layer. The isolation region includes lower and upper isolation regions and is formed from a surface of the first conductivity type layer to a surface of the semiconductor substrate and partitions the first conductivity type and second conductivity type layers into a plurality of regions. Arms of strip portions of at least one of the lower and upper isolation regions are narrowed in width toward a center of an intersection of the strip portions in a section around the intersection.
According to the semiconductor device, since arms of strip portions of at least one of the lower isolation region and the upper isolation region are narrowed in width toward a center of an intersection of region strips of the at least one in a section around the intersection, particularly, at least one of depletion layers at the interfaces between the first conductivity type semiconductor substrate and the second conductivity layer, and between the second conductivity layer and the first conductivity layer can expand more compared with a conventional device. With the expanded depletion layers, even when illumination with a laser light beam covers a section around the intersection of the strip portions of each of the upper and lower isolation regions, no electron-hole pairs generated in the depletion layer are in situ recombined under an electric field in the depletion layer expanding close to the section around the intersection of the strip portions thereof, which can contribute to generation of photovoltage.
Arms of strip portions of each of the lower and upper isolation regions are preferably narrowed in width toward a center of an intersection of the strip portions in a section around the intersection.
In this case, both depletion layers can expand toward sections around intersections of strip portions of the upper and lower isolation regions, respectively. With expansion of the depletion layers, even when illumination with a laser light beam covers the section around the intersection of the strip portions of each of the upper and lower isolation regions, no electron-hole pairs generated in the depletion layer are in situ recombined under an electric field in the depletion layer expanding close to the section around the intersection of the strip portions thereof, which can contribute to generation of photovoltage.
A third semiconductor device of an aspect of the present invention, including an isolation region having strip portions that intersect with each other in a plane, is further provided with a first conductivity type semiconductor substrate, a second conductivity type layer and the first conductivity type layer. The second conductivity type layer is formed on the semiconductor substrate. The first conductivity type layer is formed on the second conductivity type layer. The isolation region is formed from a surface of the second conductivity type layer to a surface of the semiconductor substrate, and partitions the first conductivity type and second conductivity type layers into a plurality of regions. Fore-ends of arms of strip portions of an isolation region directed to the center of an intersection of the strip portions thereof are integrally connected with one another in the vicinity of sides of the fore-ends of the arms in a section around the interconnection.
According to the semiconductor device, since the fore-ends directed to the center of an intersection of strip portions of the isolation region are integrally connected with one another in the vicinity of sides of the fore-ends of the arms in a section around the intersection, the depletion layers at the interfaces between the first and second conductivity type layers, and between the first conductivity type semiconductor substrate and the second conductivity type layer can expand more toward the respective intersections compared with a conventional semiconductor device. With the expanded depletion layers, even when illumination with a laser light beam covers the section around the intersection of the strip portions of the isolation region, no electron-hole pairs generated in the depletion layers are in situ recombined under electric fields in the depletion layers expanding close to the section around the intersection of the strip regions thereof, which can contributes to generation of photovoltage.
A fabrication process for a semiconductor device of a second aspect of the present invention is a fabrication process for a semiconductor device including an isolation region having strip portions that intersect with each other in a plane, includes the following steps of: forming a second conductivity type layer on a first conductivity type semiconductor substrate; forming, in said second conductivity type layer, a first conductivity type region to serve as an isolation region for partitioning the second conductivity type layer into a plurality of regions, from a surface of the second conductivity type layer to a surface of the semiconductor substrate; and forming a first conductivity type layer on and in the vicinity of the second conductivity type layer partitioned by the first conductivity type region, wherein the step of forming the first conductivity type region includes a step of forming the first conductivity type region such that fore-ends of respective opposed arms of each strip portion are spaced by a predetermined gap in the vicinity of a designed intersection of the strip portions of the isolation region. The predetermined gap is a distance over which the isolation region is partially formed connected between the fore-ends of the opposed arms after a final step of the fabrication process is completed.
According to this fabrication process, since when the first conductivity type region to serve as an isolation region is formed, a distance with which the fore-ends of opposed arms of respective strip regions is spaced, in a section around a designed intersection of the strip portions of the isolation region, so as to be equal to one over which said isolation region is partially formed connected between the fore-ends of the opposed arms after a final step of the fabrication process is completed, therefore the isolation region can be prevented from being larger in the section around the intersection of strip portions of the isolation region compared with a conventional fabrication process for a semiconductor device. With such a process adopted, even when illumination with a laser light beam covers a section around the intersection of the strip portions of the isolation region, no electron-hole pairs generated at a depletion layer are in situ recombined under an electric field in the depletion layer expanding close to the section around the intersection of the strip regions thereof, thus making it possible to attain a semiconductor device to efficiently generate photovoltage.
The step of forming the first conductivity type region preferably includes a step of forming fore-ends of arms of strip portions thereof so as to be narrowed in width toward the center of a designed intersection of the strip portions, in the vicinity of the designed intersection.
In this case, in a section around the designed intersection of strip portions of the first conductivity type region, opposed fore-ends narrowed in width toward the respective tips of arms of the strip portions are coalesced at a final stage to integrally form an isolation region. With formation of such an isolation region, the depletion layers at the interfaces between the first and second conductivity type layer, and between the first conductivity type semiconductor substrate and the second conductivity type layer can expand toward the intersections of strip portions. As a result, even when illumination with a laser light beam covers a section around the intersections of the strip portions of the isolation region, a photovoltage is generated in a stable manner.
Further, the step of forming the first conductivity type region preferably includes a step of forming the first conductivity type region such that fore-ends of arms of strip portions thereof directed to the center of an designed intersection of the strip portions of an isolation region are integrally connected with one another in the vicinity of sides of the fore-ends at a final stage in a section around the designed intersection.
In this case as well, an isolation region is prevented from being larger in width in a section around a designed intersection of strip portions thereof and even when illumination with a laser light beam covers the section around the intersection of the strip portions of the isolation region, such prevention of the isolation region from being larger can contributes to generation of photovoltage.
Further, when the first conductivity type region is formed as two regions of a lower region and an upper region separately, the lower region formed earlier receives an influence of a heat treatment more than the upper region formed later. For this reason, gaps between the opposed fore-ends of arms of strip portions of the lower regions are desirably set to be larger in width than ones between the fore-ends of opposed arms of strip portions of the upper region.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.